1. Field of the Invention
The invention relates to an electronic circuit for detecting a phase shift between two cyclic signals and more specifically to such a circuit for monitoring the condition of a pneumatic tire of a vehicle.
2. Description of the Prior Art
The many undesirable effects of low tire pressure are well known and many systems have been devised to warn a vehicle operator of low pressure conditions. Several prior art systems employ the phenomenon of induction to provide a signal which indicates a low tire pressure condition. In one such system, a magnet is mounted on a stationary part of the vehicle; the magnet is moved by a pressure responsive mechanism into a position for inducing a signal into the coil when the tire pressure decreases; and an electronic circuit senses the induced signal and activates a warning light. However, this system requires a relatively expensive mechanism to move the magnet and is susceptible to jamming due to the severe environmental conditions. In another such system, two annular coils are mounted on a stationary part of a vehicle and inductively separated by an annular coil concentrically mounted around the entire periphery of a wheel; the wheel mounted coil inductively couples the two stationary coils together in response to closing of a tire pressure operated switch and thereby sets off an oscillator which activates a warning device. This system is also relatively expensive due to the size of the coils, and is vulnerable to spurious signals that could set off the oscillator.